1. Field
Embodiments of the present invention relate to an electronic parking brake installed at a vehicle, and more particularly, to an electronic parking brake which prevents shaking of a shaft configured to transmit a braking power when the braking power is transmitted.
2. Description of the Related Art
Generally, a parking brake is an apparatus for stopping a vehicle not to be moved when the vehicle is parked, and serves to prevent a wheel of the vehicle from being rotated.
Recently, an electronic parking brake system which electronically controls driving of the parking brake is used. The electronic parking brake system is installed at a general disc brake and performs a function of the parking brake. Here, the electronic parking brake system is classified into a cable puller type, a motor-on-caliper type, and a hydraulic parking brake type.
FIG. 1 is a view schematically illustrating a conventional electronic parking brake. Here, the electronic parking brake system as illustrated in FIG. 1 is the motor-on-caliper type.
Referring to FIG. 1, an electronic parking brake 1 includes a disc D rotated together with a wheel (not shown) of a vehicle, a carrier 10 in which a pair of pad plates 11 and 12 disposed at both sides of the disc D to press the disc D is installed, a caliper housing 20 having a built-in piston 21 slidably installed at the carrier 10 and installed to be moved forward and backward and thus to press the pair of pad plates 11 and 12, a motor 60 configured to generate a driving force, a decelerator 40 configured to amplify the driving force generated from the motor 60, a gear assembly 50 configured to transmit the driving force of the motor 60 to the decelerator 40, and a conversion unit 30 configured to transmit a rotational force of the motor 60 to the piston 21.
The pair of pad plates 11 and 12 may be divided into an inner pad plate 11 disposed in contact with the piston 21, and an outer pad plate 12 disposed at an opposite side to the inner pad plate 11.
A cylinder 23 is provided at one side of the caliper housing 20, and the piston 21 configured to press the inner pad plate 11 toward the disc D is installed in the cylinder 23. A finger portion 22 bent downward and integrally connected with the cylinder 23 to press the outer pad plate 12 toward the disc D according to sliding movement of the caliper housing 20 is provided at the other side of the caliper housing 20.
The carrier 10 is fixed to a vehicle body and provided to prevent separation of the pair of pad plates 11 and 12 and also to guide the pair of pad plates 11 and 12 to be movable forward and backward toward the disc D.
When a braking operation is performed, the piston 21 is linearly moved through driving of the motor 60 and presses the inner pad plate 11 toward the disc D. The driving force of the motor 60 is transmitted to the decelerator 40 through a gear assembly 50, and the driving force is amplified by the decelerator 40 and then transmitted to the piston 21 through the conversion unit 30.
As described above, the conversion unit 30 serves to press the piston 21 toward the inner pad plate 11. The conversion unit 30 includes a spindle member 35 screw-coupled with the rotational shaft of the carrier 47 of the decelerator 40 to receive the rotational force of the motor 60, and a nut member 31 screw-coupled with the spindle member 35 to press the piston 21. At this time, a bearing 25 is installed in the cylinder 23 to support the spindle member 35.
The gear assembly 50 includes a driving gear 51 installed at a shaft 61 of the motor 60, a driven gear 54 connected with the decelerator 40, and a pinion idle gear 52 configured to connect the driving gear 51 and the driven gear 54. That is, the rotational force generated by rotation of the shaft 61 of the motor 60 is transmitted to the driven gear 54 through the pinion idle gear 52 engaged between the driving gear 51 and the driven gear 54.
Meanwhile, as described above, the gear assembly 50 may be formed to transmit the rotational force of the motor 60 through a plurality of gears, or formed to transmit the rotational force through a belt pulley (not shown).
The decelerator 50 is formed in a double planetary gear type. That is, the decelerator 40 includes a first deceleration part, a second deceleration part and an internal gear 44.
The first deceleration part includes a first sun gear 41 installed at a center shaft 53 of the driven gear 54, a plurality of first planetary gears 42 disposed around the first sun gear 41 to be engaged with the first sun gear 41, and a first carrier 43 connected with shafts 42a of the first planetary gears 42.
The second deceleration part has the same structure as the first deceleration part. That is, the second deceleration part includes a second sun gear 45 installed at a rotational shaft of the first carrier 43, a plurality of second planetary gears 46 disposed around the second sun gear 45 to be engaged with the second sun gear 45, and a second carrier 47 connected with shafts 46a of the second planetary gears 46. A rotational shaft of the second carrier 47 is connected with the conversion unit 30. At this time, the first and second planetary gears 42 and 46 are engaged with the internal gear 44 fixed to an outer side.
That is, in the electronic disc brake 1 as described above, the rotational force is transmitted to the decelerator 40 through the gear assembly 50 by an operation of the motor 60, and thus when the first sun gear 41 is rotated, the second planetary gears 42 engaged with the fixed internal gear 44 is idled, and the idling of the second planetary gears 42 is transmitted to the second deceleration part through the first carrier 43. Further, the second deceleration part transmits the rotational force to the spindle member 35 through the same operation as the first deceleration part, and thus decelerating rotation of the spindle member 35 is performed. When the spindle member 35 is rotated, the nut member 31 is moved in an axial direction, and presses the piston 21, and thus the braking operation is performed.
However, the electronic parking brake 1 as described above has a structure, i.e., a U-shaped power transmission structure in which the driving force of the motor 60 is primarily decelerated through the gear assembly 50 or the belt pulley structure, finally secondarily decelerated through the decelerator 40 formed in the double planetary gear type, and then converted into a linear force by the conversion unit 30, thereby generating the braking force. Therefore, when the disc brake is installed, sizes of the cylinder 23, the carrier 10 and the power transmission means (the motor, the gear assembly and the decelerator) becomes larger, and thus there is a limitation in which the electronic parking brake 1 should be installed at a medium or more-sized vehicle.
Further, when the braking operation is performed, a reaction force in an axial direction of the motor is applied by the multi-stage gears, and an alignment of the whole brake system is in disorder, and a noise may be generated at the side of the motor or the decelerator, and also a life span of the motor may be reduced.
Therefore, various researches and developments has been carried out to improve availability of an installation space of the electronic parking brake which automatically operates the brake using the motor, reduction of the noise, or the like.